The present invention relates to a method and apparatus for controlling emissions of NOx in the flue gas from systems such as combustion and fluid catalytic cracking unit (FCCU) operations. More particularly, it is an improved wet scrubbing method and apparatus that utilizes a relatively mist-free or water-droplet free ozone injection and retention zone.
The Clean Air Act Amendments were enacted in 1990 to regulate and reduce the amount of harmful pollutants released into the atmosphere. The Clean Air Act places strict guidelines on the amount of pollutants present in discharged flue gases from combustion sources and chemical plants. As a result, industries seek to implement efficient, but cost-effective systems and methods for controlling the release of harmful pollutants.
Nitrogen oxides (NOx) and sulfur oxides (SOx) are primary combustion pollutants targeted for reduction by the Clean Air Act. Various treatment processes and methods have been developed to reduce NOx in a flue gas. NOx reduction technologies employed for reducing the NOx concentration in the flue gas were typically either selective catalytic reduction (SCR) or non-catalytic reduction (SNCR) using ammonia or urea to convert the NOx to elemental nitrogen.
In the SNCR process, ammonia reduces NOx present in the flue gas to nitrogen, without the presence of a catalytic substance. The ammonia is then oxidized to nitrogen and water. The SNCR process is significantly limited, however, by the fact that it requires a very high operational temperature, ranging from 800°-1200° C. Even at these high operational temperatures, the percentage of NOx reduction is limited to about 60% and lower in most cases.
The SCR process requires the presence of a catalytic converter during the contact between the flue gas and ammonia or ammonia precursor. The process can be carried out at temperatures significantly lower than the SNCR process, such as 250°-600° C. However, the SCR process also has limitations.
The SCR process is more costly than the SNCR process, as the catalyst deteriorates with age and requires replacement every 3-5 years. Additionally, the catalyst can convert some of the SO2 present in the flue gas to SO3, which may cause additional pollution concerns. Lastly, in certain applications the catalyst bed may trap particulate matter causing blinding of the catalyst and/or a build-up of flue-gas pressure drop. As a result, a particulate removal device such as an electrostatic precipitator often precedes SCR.